Light Manipulation in Organic Photovoltaics

Water-Based Additive-Free Chromic Inks for Printing of Flexible Photochromics and Electrochromics

Digital inkjet printing has become one of the most convenient and efficient technologies for coating chromic materials on flexible substrates with complicated patterns. However, the development of water-based, additive-free chromic inks for inkjet printing still remains a challenge. Herein, three ammonium-functionalized colorless viologen derivatives AV, APV, and AQV with excellent water solubilities are utilized as chromes in the chromic inks due to their excellent photochromic and electrochromic properties. Water, ethanol, and ethylene glycol are selected as cosolvents, and their contents in this ternary solvent system have been optimized to achieve comprehensive rheological properties. With the H2O:EtOH:EG weight ratio of 8:1:7, the chromic ink based on AV realizes a viscosity of 4.69 mPa·s, a surface tension of 45.13 mN/m, and a Z value of 3.87. Without adding any additive, the as-prepared chromic inks can be printed on flexible substrates, such as paper and poly(ethylene terephthalate) (PET) films, by a conventional inkjet printer with inherent high resolutions. The printed patterns are initially invisible due to the colorless characteristics of the chromic inks. Interestingly, the printed films are responsive to both light and electric stimuli. Upon irradiation by UV light, a series of sentences with font sizes from 5 to 12 points and four quick response codes with different lattice resolutions clearly appear on the printed paper. Meanwhile, after printing on an indium tin oxide-coated PET substrate, electrochromic devices (ECDs) can be facilely fabricated by covering a hydrogel electrolyte on the printed films. Upon application of different potentials, the assembled ECDs exhibit "Peking Opera facial makeup" patterns with different colors. Therefore, the developed water-based additive-free chromic inks can be utilized for information display and encryption applications.

Organic Solar Cells Improved by Optically Resonant Silicon Nanoparticles

Silicon nanophotonics has become a versatile platform for optics and optoelectronics. For example, strong light localization at the nanoscale and lack of parasitic losses in infrared and visible spectral ranges make resonant silicon nanoparticles a prospect for improvement in such rapidly developing fields as photovoltaics. Here, we employed optically resonant silicon nanoparticles produced by laser ablation for boosting the power conversion efficiency of organic solar cells. Namely, we created colloidal solutions of spherical nanoparticles with a range of diameters (80-240 nm) in different solvents. We tested how the nanoparticles' position in the device, their concentration, silicon doping, and method of deposition affected the final device efficiency. The best conditions optimization resulted in an efficiency improvement from 6% up to 7.5%, which correlated with numerical simulations of nanoparticles' optical properties. The developed low-cost approach paves the way toward highly efficient and stable solution-processable solar cells.

The effect of positional disorder and the Beer-Lambert law in organic photovoltaics : A kinetic Monte Carlo simulation analysis

It is urgent to address climate change by radically changing our energy sources. Organic photovoltaics (OPVs) are a competitive clean energy emerging technology and will undoubtedly have a market niche in a world that needs to take advantage of every possible type of renewable energy. Recent studies have brought relevant improvements on internal efficiency, focusing on two properties at the interface: energetic disorder and bending. However, how positional disorder affects internal efficiency is still an open question. Here, we show that positional disorder is desired at the interface, but only up to a threshold value of 0.2 nm for poly p-phenylene vinylene. Using a kinetic Monte Carlo simulator, we realized that not enough excitons were reaching the interface, and introduced the Beer-Lambert law of attenuance to correct it. Furthermore, we realized that the same disorder that facilitates charge separation at the interface diminishes exciton and charge mobility in bulk, so we propose here a new morphology for the active layer of OPVs. Our suggestion implicates in better overall performance, improving not just the internal but the overall cell efficiency.

A Review on the Materials Science and Device Physics of Semitransparent Organic Photovoltaics

In this review, the current state of materials science and the device physics of semitransparent organic solar cells is summarized. Relevant synthetic strategies to narrow the band gap of organic semiconducting molecules are outlined, and recent developments in the polymer donor and near-infrared absorbing acceptor materials are discussed. Next, an overview of transparent electrodes is given, including oxides, multi-stacks, thin metal, and solution processed electrodes, as well as considerations that are unique to ST-OPVs. The remainder of this review focuses on the device engineering of ST-OPVs. The figures of merit and the theoretical limitations of ST-OPVs are covered, as well as strategies to improve the light utilization efficiency. Lastly, the importance of creating an in-depth understanding of the device physics of ST-OPVs is emphasized and the existing works that answer fundamental questions about the inherent changes in the optoelectronic processes in transparent devices are presented in a condensed way. This last part outlines the changes that are unique for devices with increased transparency and the resulting implications, serving as a point of reference for the systematic development of next-generation ST-OPVs.

Optical Model and Optimization for Coherent-Incoherent Hybrid Organic Solar Cells with Nanostructures

Embedding nanostructures in organic solar cells (OSCs) is a well-known method to improve the absorption efficiency of the device by introducing the plasma resonance and scattering effects without increasing the active layer thickness. The introduction of nanostructures imposes greater demands on the optical analysis method for OSCs. In this paper, the generalized rigorous coupled-wave analysis (GRCWA) is presented to analyze and optimize the performance of coherent-incoherent hybrid organic solar cells (OSCs) with nanostructures. Considering the multiple reflections of light scattered within the glass substrate by the device, the correction vector g is derived, then the modified expressions for the field and absorption distribution in OSCs are provided. The proposed method is validated by comparing the simulated results of various structures with results obtained by the generalized transfer matrix method (GTMM) and the “equispaced thickness method” (ETM). The results demonstrate that the proposed method can reduce the number of simulations by at least half compared to the ETM while maintaining accuracy. With the proposed method, we discussed the device performance depending on the geometrical parameters of nanostructures, and the optimization and analysis are accomplished for single and tandem OSCs. After optimization based on the proposed method, the performance of OSCs are significantly improved, which further demonstrates the practicality of the method.

Characterization of optical manipulation using microlens arrays depending on the materials and sizes in organic photovoltaics

Various physical structures have improved light-harvesting and power-conversion efficiency in organic photovoltaic devices, and optical simulations have supported the improvement of device characteristics. Herein, we experimentally investigated how microlens arrays manipulate light propagation in microlens films and material stacks for organic photovoltaics to understand the influence of the constituent materials and sizes of the microlens. As materials to fabricate a microlens array, poly(dimethylsiloxane) and Norland Optical Adhesive 63 were adopted. The poly(dimethylsiloxane) microlens array exhibited higher total transmittance and higher diffuse transmittance, further enhancing the effective optical path and light extinction in material stacks for organic photovoltaics. This resulted in more current generation in an organic photovoltaic device with a poly(dimethylsiloxane) microlens array than in a Norland Optical Adhesive 63 microlens array. The sizes of the microlenses were controlled from 0.5 to 10 μm. The optical characteristics of microlens array films and material stacks with microlenses generally increased with size of the microlens, leading to a 10.6% and 16.0% improvement in the light extinction and power-conversion efficiency, respectively. In addition, electron and current generation in material stacks for organic photovoltaics were calculated from light extinction. The theoretical current generation matched well with experimental values derived from organic photovoltaic devices. Thus, the optical characterization of physical structures helps to predict how much more current can be generated in organic photovoltaic cells with a certain physical structure; it can also be used for screening the physical structures of organic photovoltaic cells.

The influence of constituent materials and sizes of a microlens was experimentally and theoretically explored.

Numerical study on the angular light trapping of the energy yield of organic solar cells with an optical cavity

A limiting factor in organic solar cells (OSCs) is the incomplete absorption in the thin absorber layer. One concept to enhance absorption is to apply an optical cavity design. In this study, the performance of an OSC with cavity is evaluated. By means of a comprehensive energy yield (EY) model, the improvement is demonstrated by applying realistic sky irradiance, covering a wide range of incidence angles. The relative enhancement in EY for different locations is found to be 11-14% compared to the reference device with an indium tin oxide front electrode. The study highlights the improved angular light absorption as well as the angular robustness of an OSC with cavity.

Absorption enhancement by semi-cylindrical-shell-shaped structures for an organic solar cell application

Organic solar cells are attractive for various applications with their flexibility and low-cost manufacturability. In order to increase their attractiveness in practice, it is essential to improve their energy conversion efficiency. In this work, semi-cylindrical-shell-shaped structures are proposed as one of the approaches, aiming at absorption enhancement in an organic solar cell. Poly(3-hexylthiophene-2,5-diyl) blended with indene-C60 bisadduct (P3HT:ICBA) is considered as the active layer. Light coupling to the guided modes and a geometrical advantage are attributed to this absorption enhancement. Finite-difference time-domain methods and finite element analysis are used to examine the absorption spectra for two types of devices, i.e., a debossed type and an embossed type. It is shown that absorption enhancement increases as the radius of the cylinder increases, but reaches a saturation at about 4-µm radius. The average absorption enhancement with an active layer thickness of 200 nm and radius of 4 µm, and for incidence angles between 0° and 70°, is found as 51%-52% for TE-polarized input and as 30%-33% for TM-polarized input when compared to a flat structure. Another merit of the proposed structures is that the range of incidence angles where the integrated absorption is at the level of the normal incidence is significantly broadened, reaching 70°-80°. This feature can be highly useful especially when organic solar cells are to be placed around a round object. The study results also exhibit that the proposed devices bear broadband absorption characteristics.

Plasmon-Assisted Direction- and Polarization-Sensitive Organic Thin-Film Detector

Utilizing Bragg surface plasmon polaritons (SPPs) on metal nanostructures for the use in optical devices has been intensively investigated in recent years. Here, we demonstrate the integration of nanostructured metal electrodes into an ITO-free thin film bulk heterojunction organic solar cell, by direct fabrication on a nanoimprinted substrate. The nanostructured device shows interesting optical and electrical behavior, depending on angle and polarization of incidence and the side of excitation. Remarkably, for incidence through the top electrode, a dependency on linear polarization and angle of incidence can be observed. We show that these peculiar characteristics can be attributed to the excitation of dispersive and non-dispersive Bragg SPPs on the metal–dielectric interface on the top electrode and compare it with incidence through the bottom electrode. Furthermore, the optical and electrical response can be controlled by the organic photoactive material, the nanostructures, the materials used for the electrodes and the epoxy encapsulation. Our device can be used as a detector, which generates a direct electrical readout and therefore enables the measuring of the angle of incidence of up to 60° or the linear polarization state of light, in a spectral region, which is determined by the active material. Our results could furthermore lead to novel organic Bragg SPP-based sensor for a number of applications.

Using Dual Microresonant Cavity and Plasmonic Effects to Enhance the Photovoltaic Efficiency of Flexible Polymer Solar Cells

Fabricating polymer solar cells (PSCs) on flexible polymer substrates, instead of on hard glass, is attractive for implementing the advantage and uniqueness of the PSCs represented by mechanically rollable and light-weight natures. However, simultaneously achieving reliable robustness and high-power conversion efficiency (PCE) in such flexible PSCs is still technically challenging due to poor light harvesting of thin photoactive polymers. In this work, we report a facile, effective strategy for improving the light-harvesting performance of flexible PSCs without sacrificing rollability. Very high transparent (93.67% in 400–800 nm) and low sheet resistance (~10 Ω sq⁻¹) ZnO/Ag_(O)/ZnO electrodes were implemented as the flexible substrates. In systematically comparison with ZnO/Ag/ZnO electrodes, small amount of oxygen induced continuous metallic films with lower thickness, which resulted in higher transmittance and lower sheet resistance. To increase the light absorption of thin active layer (maintain the high rollability of active layer), a unique platform simultaneously utilizing both a transparent electrode configuration based on an ultrathin oxygen-doped Ag, Ag_(O), and film and plasmonic Ag@SiO₂ nanoparticles were designed for fully leveraging the advantages of duel microresonant cavity and plasmonic effects to enhance light absorbance in photoactive polymers. A combination of the ZnO/Ag_(O)/ZnO electrode and Ag@SiO₂ nanoparticles significantly increased the short-current density of PSCs to 17.98 mA cm⁻² with enhancing the photoluminescence of PTB7-Th film. The flexible PSC using the optimized configuration provided an average PCE of 8.04% for flexible PSCs, which was increased by 36.27% compared to that of the PSC merely using a conventional transparent indium tin oxide electrode.

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

Hybrid nanostructures, composed of multi-component crystals of various shapes, sizes and compositions are much sought-after functional materials. Pairing the ability to tune each material separately and controllably combine two (or more) domains with defined spatial orientation results in new properties. In this review, we discuss the various synthetic mechanisms for the formation of hybrid nanostructures of various complexities containing at least one metal/semiconductor interface, with a focus on colloidal chemistry. Different synthetic approaches, alongside the underlying kinetic and thermodynamic principles are discussed, and future advancement prospects are evaluated. Furthermore, the proved unique properties are reviewed with emphasis on the connection between the synthetic method and the resulting physical, chemical and optical properties with applications in fields such as photocatalysis.

Coatings in Photovoltaic Solar Energy Worldwide Research

This paper describes the characteristics of contributions that were made by researchers worldwide in the field of Solar Coating in the period 1957–2019. Scopus is used as a database and the results are processed while using bibliometric and analytical techniques. All of the documents registered in Scopus, a total of 6440 documents, have been analyzed and distributed according to thematic subcategories. Publications are analyzed from the type of publication, field of use, language, subcategory, type of newspaper, and the frequency of the keyword perspectives. English (96.8%) is the language that is most used for publications, followed by Chinese (2.6%), and the rest of the languages have a less than < 1% representation. Publications are studied by authors, affiliations, countries of origin of the authors, and H-index, which it stands out that the authors of China contribute with 3345 researchers, closely followed by the United States with 2634 and Germany with 1156. The Asian continent contributes the most, with 65% of the top 20 affiliations, and Taiwan having the most authors publishing in this subject, closely followed by Switzerland. It can be stated that research in this area is still evolving with a great international scientific contribution in improving the efficiency of solar cells.

Recent progress of light manipulation strategies in organic and perovskite solar cells

Organic and perovskite solar cells are suffering from the insufficient utilization of incident light and thus low light harvesting efficiency despite their rapid progress in the past decade. In this regard, light manipulation strategies have attracted numerous attention to solve this inherent limit. Herein, the recent advances in light manipulation techniques in this area are overviewed. The light manipulation mechanisms are illustrated to classify the structures. Various light manipulation structures, fabrication techniques, and corresponding results are given and discussed, addressing the suppression of surface reflection, nano/micro-structure-induced light scattering, and the plasmonic effects with periodic metallic patterns and metallic nanoparticles. A brief perspective on future research is also proposed for pursuing broadband light harvesting.

Control of Femtoliter Liquid on a Microlens: A Way to Flexible Dual-Microlens Arrays

Microlens arrays are key elements for light management in optoelectronic devices. The recent advancement in the wearable intelligent electronics has driven the development of flexible microlenses. In this work, we show a controllable and scalable surface-droplet-based strategy to create unconventional flexible polymer microlens arrays. The technique is underpinned by the morphological transition of femtoliter liquid on the surface of a microlens surrounded by a planar area. We found that the droplet liquid wetted the rim of the microlens first and gradually moved upward to the microlens surface with an increase in the liquid volume. The morphology evolution of the droplet is in good agreement with the predication from our simulations based on the interfacial energy minimization under the condition of the pinned boundary. The shape of the droplet on the microlens is well controlled by the droplet volume, aspect ratio of the microlens, and the interfacial energy of the droplets on the microlens. As a result, the obtained structures of one microlens partially covered by a droplet can be produced in arrays over a large scale, serving as templates for fabricating transparent polymer double microlens arrays for improved light emission from the optoelectronic device.

Flexible Perovskite Solar Cells via Surface-Confined Silver Nanoparticles on Transparent Polyimide Substrates

We report about a flexible substrate incorporating surface-confined silver nanoparticles on transparent polyimide (PI). The incorporated silver nanoparticles (Ag NPs), which possessed excellent adhesive strength with the PI substrate, induced localized surface plasmon resonance and light scattering effects by changing the particle size and interparticle distance to promote light harvesting in the perovskite solar cells. Moreover, the reduced sheet resistance was beneficial for the charge extraction and transportation in the devices when high-conductivity poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, PH1000) was deposited on the Ag NP-confined PI serving as a flexible bottom electrode. A power conversion efficiency of 10.41% was obtained for the flexible perovskite solar cells based on a Ag NP-confined PI substrate (the particle size of the Ag NPs was 25 nm mixed with 40 nm), which was obviously enhanced in all parameters. Especially, a 61% improvement existed in the short-circuit current density compared to that based on the bare PI substrates. It indicates that the substrate would be a promising candidate for the development of flexible electronics.

Pinhole-free TiO2/Ag(O)/ZnO configuration for flexible perovskite solar cells with ultralow optoelectrical loss

Perovskite solar cells (PSCs) fabricated on transparent polymer substrates are considered a promising candidate as flexible solar cells that can emulate the advantages of organic solar cells, which exhibit considerable freedom in their device design thanks to their light weight and mechanically flexibility while achieving high photocurrent conversion efficiency, comparable to that of their conventional counterparts fabricated on rigid glasses. However, the full realization of highly efficient, flexible PSCs is largely prevented by technical difficulties in simultaneously attaining a transparent electrode with efficient charge transport to meet the specifications of PSCs. In this study, an effective strategy for resolving this technical issue has been devised by proposing a simple but highly effective technique to fabricate an efficient, multilayer TiO₂/Ag_(O)/ZnO (TAOZ) configuration. This configuration displays low losses in optical transmittance and electrical conductivity owing to its completely continuous, ultrathin metallic Ag_(O) transparent electrode, and any notable current leakage is suppressed by its pinhole-free TiO₂ electron transport layer. These features are a direct consequence of the rapid evolution of Ag_(O) and TiO₂ into ultrathin, completely continuous, pinhole-free layers owing to the dramatically improved wetting of metallic Ag_(O) with a minimal dose of oxygen (ca. 3 at%) during sputtering. The TAOZ configuration exhibits an average transmittance of 88.5% in the spectral range of 400–800 nm and a sheet resistance of 8.4 Ω sq⁻¹ while demonstrating superior mechanical flexibility to that of the conventional TiO₂ on ITO configuration. The photocurrent conversion efficiency of flexible PSCs is significantly improved by up to 11.2% thanks to an optimum combination of optoelectrical performance and pinhole-free morphologies in the TAOZ configuration.

A TiO₂/Ag_(O)/ZnO configuration is developed for flexible perovskite solar cells to provide a pinhole-free electron transport layer and a transparent electrode.

Broadband light trapping strategies for quantum-dot photovoltaic cells (>10%) and their issues with the measurement of photovoltaic characteristics

Bandgap tunability and broadband absorption make quantum-dot (QD) photovoltaic cells (PVs) a promising candidate for future solar energy conversion systems. Approaches to improving the electrical properties of the active layer increase efficiency in part. The present study focuses on optical room for enhancement in QD PVs over wide spectrum in the near-infrared (NIR) region. We find that ray-optical light trapping schemes rather than the nanophotonics approach may be the best solution for enhancing broadband QD PVs by suppressing the escape probability of internal photons without spectral dependency. Based on the theoretical study of diverse schemes for various bandgaps, we apply a V-groove structure and a V-groove textured compound parabolic trapper (VCPT) to PbS-based QD PVs along with the measurement issues for PVs with a light scattering layer. The efficiency of the best device is improved from 10.3% to 11.0% (certified to 10.8%) by a V-groove structure despite the possibility of underestimation caused by light scattering in small-area devices (aperture area: 0.0625 cm²). By minimizing such underestimation, even greater enhancements of 13.6% and 15.6% in short circuit current are demonstrated for finger-type devices (0.167 cm² without aperture) and large-area devices (2.10 cm² with an aperture of 0.350 cm²), respectively, using VCPT.

Understanding the Effects of a High Surface Area Nanostructured Indium Tin Oxide Electrode on Organic Solar Cell Performance

Organic solar cells (OSCs) are a complex assembly of disparate materials, each with a precise function within the device. Typically, the electrodes are flat, and the device is fabricated through a layering approach of the interfacial layers and photoactive materials. This work explores the integration of high surface area transparent electrodes to investigate the possible role(s) a three-dimensional electrode could take within an OSC, with a BHJ composed of a donor-acceptor combination with a high degree of electron and hole mobility mismatch. Nanotree indium tin oxide (ITO) electrodes were prepared via glancing angle deposition, structures that were previously demonstrated to be single-crystalline. A thin layer of zinc oxide was deposited on the ITO nanotrees via atomic layer deposition, followed by a self-assembled monolayer of C₆₀-based molecules that was bound to the zinc oxide surface through a carboxylic acid group. Infiltration of these functionalized ITO nanotrees with the photoactive layer, the bulk heterojunction comprising PC₇₁BM and a high hole mobility low band gap polymer (PDPPTT-T-TT), led to families of devices that were analyzed for the effect of nanotree height. When the height was varied from 0 to 50, 75, 100, and 120 nm, statistically significant differences in device performance were noted with the maximum device efficiencies observed with a nanotree height of 75 nm. From analysis of these results, it was found that the intrinsic mobility mismatch between the donor and acceptor phases could be compensated for when the electron collection length was reduced relative to the hole collection length, resulting in more balanced charge extraction and reduced recombination, leading to improved efficiencies. However, as the ITO nanotrees increased in height and branching, the decrease in electron collection length was offset by an increase in hole collection length and potential deleterious electric field redistribution effects, resulting in decreased efficiency.

High-Performance Inverted Polymer Solar Cells with Zirconium Acetylacetonate Buffer Layers

Inverted polymer solar cells incorporating solution-processed zirconium acetylacetonate (ZrAcac) buffer layers were demonstrated. The optimal device delivered a power conversion efficiency up to 9.2%, displaying ∼20% improvement compared with the device of conventional configuration. The performance improvement by adopting ZrAcac as the cathode buffer layer is attributed to the enhanced light-harvesting, facilitated electron transport, and reduced bimolecular recombination loss. The morphology of ZrAcac buffer layer was found to be critical in achieving high performance, which was tunable through the selection of processing solvents. A flat and uniform ZrAcac film consisting of ∼20 nm nanoscale aggregates deposited from a chloroform solution was proved to be highly effective, which only requires a short light-soaking time.